This invention relates generally to sonar systems, and more particularly to a method and apparatus for imaging underwater targets by utilizing sidelooking sonar.
In a forward looking sonar, targets give no appreciable doppler information; imaging may be done at close range using beam steering information. In a sidelooking sonar, differences in azimuthal position of target highlights map into differences in doppler shift, allowing determination of both range and crossrange information.
One method of imaging using a sidelooking sonar involves transmission of a signal with a large time-bandwidth product to achieve good range and crossrange resolution simultaneously. Extensive calculation is required to compute a two-dimensional range doppler map, as well as to process the resulting image. Also, such wideband signals invites violation of the narrowband assumptions of the underlying scattering function theory.
An alternate method is to transmit a set of subpulses and compare how well their respective ambiguity functions match the target's scattering function; this method works only for line targets consisting of a dense collection of uniform-strength scatterers.